1. Field of the Invention
The present invention relates to cranking systems for motor vehicle engines.
2. Description of the Related Art
A conventional starter motor assembly 20 for a motor vehicle engine is illustrated in FIG. 6. Starter motor assembly 20 includes cranking motor 22 and starter solenoid 24. Cranking motor 22 includes drive assembly 26 which typically includes an overrunning clutch and which further includes pinion gear 28. Drive assembly 26 is translatably mounted on shaft 30 such that when translated to the right as viewed in FIG. 6, pinion gear 28 can mesh with a ring gear 32 on the engine. When pinion gear 28 and ring gear 32 are so meshed, cranking motor 22 can crank the engine.
Starter solenoid 24 includes two electrical coils, pull-in coil 34 and hold-in coil 36. Pull-in coil 34 and hold-in coil 36 are electromagnetically coupled to plunger assembly 38. The movement of plunger assembly 38 to the left as viewed in FIG. 6 during actuation of starter solenoid 24 has two effects. One, plunger assembly 38 pulls on lever 40, translating drive assembly 26 to the right such that pinion gear 28 can mesh with ring gear 32. Two, movable contact 42 electrically couples fixed contacts 44 and 46. Through this coupling, battery power is provided to cranking motor 22 for cranking the engine.
Electrically, a cranking system which employs starter motor assembly 20 is illustrated with additional reference to FIG. 7. Battery 48 provides electrical power for cranking motor 22 and starter solenoid 24. When ignition switch 50 is closed, pull-in coil 34 is energized via the armature winding of cranking motor 22. Hold-in coil 36 is also energized. Plunger assembly 38 is thus drawn to the left as viewed in FIGS. 6 and 7.
While solenoid 24 is being actuated, two alternative scenarios can occur. In one, the teeth of pinion gear 28 might be offset from the teeth of ring gear 32, allowing meshing of those two gears. In that case, the gears mesh and movable contact 42 electrically couples fixed contacts 44 and 46. This both shorts pull-in coil 34 (leaving hold-in coil 36 to hold engagement of pinion gear 28 with ring gear 32) and provides electrical power to cranking motor 22 to crank the engine.
In the second alternative scenario, the teeth of pinion gear 28 may be aligned with the teeth of ring gear 32, preventing meshing of those two gears and movement of movable contact 42 into contact with fixed contacts 44 and 46. In that event, mesh spring 49 compresses, allowing plunger assembly 38 to fully actuate, engaging movable contact 42 with fixed contacts 44 and 46. Then, pull-in coil 34 is shorted and cranking motor 22 turns, as before. As cranking motor 22 turns, the compressed mesh spring 49 forces pinion gear 28 into mesh with ring gear 32.
Timing diagrams showing the events which take place during cranking in a system using conventional starter motor assembly 20 is shown in FIG. 8. At time t.sub.0, ignition switch 50 is closed by the operator of the vehicle. The current of starter solenoid 24 includes current drawn by both pull-in coil 34 and hold-in coil 36. At time t.sub.1, movable contact 42 couples fixed contacts 44 and 46. This shorts pull-in coil 34, leaving only the current of hold-in coil 36 being drawn by solenoid 24. Also at time t.sub.1, current is provided to cranking motor 22 via movable contact 42's coupling with fixed contacts 44 and 46. This current starts at a relatively high level and decreases to a fairly steady level as cranking motor 22 gets up to speed. Finally, at time t.sub.2, ignition switch 50 has been turned off, either due to the engine having been successfully started or due to the operator of the vehicle ending the cranking event for another reason. After ignition switch 50 has been turned off, return spring 52 (FIG. 6) forces plunger assembly 38 back to the right, disengaging drive assembly 26 from ring gear 32.
A concern with the conventional cranking system illustrated in FIGS. 6-8 occurs in the aforementioned case in which pinion gear 28 interferes with ring gear 32 while solenoid 24 is actuating. In that event, mesh spring 49 does allow solenoid 24 to complete its actuation. However, when the actuation is complete, energizing cranking motor 22 and shorting pull-in coil 34, hold-in coil 36 is left alone to supervise the meshing of pinion gear 28 with ring gear 32. This can cause a less-than-robust final pull-in, causing milling of pinion gear 28 and ring gear 32. Also, relying on only hold-in coil 36 for the final pull-in makes the pull-in event more susceptible to variances in battery voltage and temperature.
Further, in the conventional cranking system of FIGS. 6-8, starter motor assembly 20 is a relatively large package. Also, by necessity, starter motor assembly 20 is usually packaged in an unfriendly environment (i.e., low in the engine compartment), where it can be exposed to dirt, water splash, road salt and high temperatures. The reliability of an electrical component such as solenoid 24, especially the reliability of contacts 42, 44 and 46, can be adversely affected by such an unfriendly environment.
A system which can overcome the several concerns detailed above with respect to a conventional cranking system can provide considerable performance and durability advantages over the conventional cranking system.